Gas burner

ABSTRACT

A gas burner capable of automatically shutting off the supply of a gas fuel when the content of the oxygen in the air supplied to the gas burner drops to a predetermined level. It incorporates a Smithell&#39;s gas burner as a pilot burner which consists of an inner tube and an outer tube formed with an auxiliary air port. The sizes of these inner and outer tubes as well as the diameter and position of the auxiliary air port are so selected that when the contents of the oxygen in the air supplied to the gas burner drops to a predetermined level, the inner flame cone at the mouth of the inner tube is blown off. A sensor is provided which detects the blown off and generates the output signal in response to which a control means such as a solenoid-operated control valve may close the gas supply pipe. The pilot burner is of the general type in that it can burn gas fuels having different heating values.

BACKGROUND OF THE INVENTION

The present invention relates to an improvement of a gas burner.

In general the conventional gas burners such as gas-fired water heatersare provided with pilot burner-safety means which can automatically shutoff the supply of a gas fuel to the gas burner when the pilot flame coneis blown off for any reason. However, this safety device cannot respondto reduction of the oxygen content in the air, below a normal level, sothat incomplete combustion results, generating a large amount of carbonmonoxide (CO) which represents a very serious hazard to the lives andhealth of people who is not aware of the incomplete combustion.

In order to overcome the above problem, there has been proposed anddemonstrated a pilot burner which is so designed and constructed thatwhen the content of the oxygen in the air supplied to the gas burnerdrops below a normal level, the flame cone may lift off the mouth of thepilot burner. The lift-off of the pilot flame cone in turn is detectedby a suitable sensor so that the deficiency of the oxygen content in theair supplied to the gas burner may be detected and in response to theoutput signal a suitable safety means such as a control valve is closedso as to interrupt the supply of gas fuel. The pilot burner of the typedescribed is effective when LNG or LPG is burned because of its slowburning velocity, but is ineffective when a town or city gas is burnedbecause it has a high burning velocity. That is, as will as described indetail below with reference to FIGS. 1-5, the response is so slow thatbefore the change in height of the pilot flame cone is detected, themain burner has generated a large amount of CO. When a city or town gasis burned, the height of the pilot flame cone is relatively high undernormal conditions; that is, when sufficient oxygen is supplied, but whenthe content of oxygen in the air drops below a normal level, the heightof the pilot flame cone is reduced so slowly that the difference betweenthe normal and abnormal heights of the pilot flame cone cannot bedetected immediately.

SUMMARY OF THE INVENTION

Accordingly, one of the objects of the present invention is to providean improved Smithell type pilot burner which consists of an inner tubeand an outer tube formed with an auxiliary air intake ports inaccordance with the present invention so that when the contents of theoxygen in the air drops below a normal level, the states of the innerand outer flame cones can be very clearly distinguished from those undernormal conditions.

Another object of the present invention is to provide an improvedSmithell type pilot burner of the type described which may be used withdiffernt gas fuels without modification.

A further object of the present invention is to provide a gas burnerwhich incorporates the pilot burner of the type described so that whenthe contents of the oxygen in the air supplied to the gas burner dropsbelow a normal level, the supply of gas fuel may be automatically andimmediately shut off, whereby poisoning by carbon monoxide may beprevented.

Briefly stated, to the above and other ends, the present inventionprovides a gas burner comprising an inner tube and an outer tubedisposed concentrically of the inner tube and spaced apart therefromboth diametrically and axially by suitable distance and formed with anauxiliary air inlet port, the sizes of the inner and outer tubes andtheir relative position as well as the size and position of theauxiliary air inlet port being so determined that under the conditionsthat the boundary velocity gradient is between 7×10² and 4×10³ sec⁻¹ andthe equivalent ratio is between 1.1 and 2.0, the mixture of the primaryair and a gas fuel admitted into the inner tube from the lower endthereof may be burned at its mouth, producing the inner or primary flamecone while the unburned mixture emerging from the inner or primary flamecone is mixed with the auxiliary air admitted through the auxiliary airinlet port and burned at the mouth of the outer tube, producing theouter or secondary flame cone.

In this specification, the term "boundary velocity gradient" is definedas the gradient of the tangent line of the flame while the term "theequivalent ratio", is (the volume of gas/the volume of air)/(the volumeof gas)/(the theoretical volume of air).

BRIEF DESCRIPTION OF THE DRAWINGS:

FIG. 1 is a front view, partly in section, of a prior art pilot burner;

FIG. 2 shows the combustion characteristic curves thereof;

FIG. 3 is a front view, in section, of another prior art pilot gasburner;

FIG. 4 shows the combustion characteristic curves thereof;

FIG. 5 shows the states of the inner and outer flame cones thereofdepending upon the amount of the primary air supplied;

FIG. 6 is a perspective view of a first embodiment of a gas burner inaccordance with the present invention;

FIG. 7 shows the states of the inner and outer flame cones thereofdependent upon the amount of the primary air supplied;

FIG. 8 is a graph used for the explanation thereof;

FIG. 9 is a longitudinal sectional view thereof;

FIG. 10 is a graph used for the explanation of the relationship betweenthe inner diameter of the inner tube and the length or height of theflame cone produced at the mouth thereof;

FIG. 11 is a graph used for the explanation of the relationship betweenthe inner diameters of the inner and outer tubes;

FIG. 12 is a graph illustrating the relationship between the oxygen andthe electromotive force generated by a thermocouple which is used as anoxygen content sensor;

FIG. 13 shows the difference in shape between the outer and inner flamecones depending upon the distance between the mouths of the outer andinner tubes;

FIG. 14 shows the relationship between the height of the inner flamecone and the electromotive force generated by the thermocouple;

FIG. 15 shows the relationship between the distance between the mouthsof the inner and outer tubes on the one hand and the electromotive forcegenerated by the thermocouple when the oxygen contents is 18%;

FIG. 16 shows the relationship between the diameter of the auxiliary airinlet port and the electromotive force generated by the thermocouplewith the oxygen contents as a parameter when a gas fuel having a highestburning velocity is burned;

FIG. 17 is a graph similar to FIG. 16 but when a gas fuel with a slowestburning velocity is burned;

FIG. 18 shows the relationship between the oxygen contents in % and theelectromotive force generated by the thermocouple with the position ofthe auxiliary air inlet port as a parameter;

FIG. 19 shows the relationship between the length of the inner tube andthe electromotive force generated by the thermocouple with the oxygencontents as a parameter;

FIG. 20 shows a gas-fired water heater incorporating a pilot gas burnerin accordance with the present invention;

FIG. 21 shows the relationship between the distance between the primaryflame cone and the thermocouple and the electromotive force generated bythe latter;

FIG. 22 is a schematic front view of a second embodiment of the presentinvention; and

FIG. 23 is a top view thereof.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Prior Art, FIGS. 1-5

Prior to the description of the preferred embodiments of the presentinvention, the prior art gas burner will be described briefly in orderto more definitely and specifically point out the problems thereof. InFIG. 1 is shown a pilot burner comprising a nozzle 1 and a combustiontube 2 which is fitted over the nozzle 1 and extended upwardly and isformed with an air hole 3. The air admitted through this air hole 3serves to lift the flame through the combustion tube 2. When the supplyof the oxygen is not sufficient, the flame rises above a predeterminedheight, whereby the oxygen deficiency may be detected. This scheme orarrangement is satisfactory in the case of LNG and LPG which has a lowcombustion speed, but is unsatisfactory or unreliable in the case of thetown or city gas which has a high combustion speed. More particularlybefore the flame of the pilot burner lifts so as to indicate theinsufficient supply of the oxygen, the main burner produces a largequantity of CO. As shown in FIG. 2, the town or city gases have a highcombustion speed and a high flame lift which is very slow to vary inresponse to the amount of the oxygen supplied. As a result, even in thecase of the deficiency of the oxygen, the flame lift does not fallquickly so that it becomes very ambiguous to detect whether a sufficientamount of the oxygen is supplied only in terms of the flame lift.

As shown in FIG. 3, there has been devised and demonstrated a gas burnerof the type wherein the primary flame and the secondary flame may beseparated from each other so that the states of the flames may be variedover a wide range depending upon the supply of oxygen even when town orcity gas having a high combustion speed is used. This gas burnercomprises a main body 6 consisting of an inner tube 4 and an outer tube5. The mouth or the flameholder 8 of the outer tube 5 is verticallyupwardly spaced apart from the mouth or the flameholder 7 of the innertube 4 by a suitable distance. The inner tube 4 has a nozzle 9 fitted atthe lower end thereof and is formed with air ports 10. The gas suppliedthrough the nozzle 9 and the primary air admitted through the air ports10 are mixed and burned to form the inner or primary cone of flame F₁while the unburned gas burns at the mouth 8 of the outer tube to formthe outer or secondary cone of flame F₂. It is well known in the artthat in a Smithell's burner, wherein the inner and outer flame cones F₁and F₂ are separated from each other as described above, the state ofthe inner or primary flame cone F₁ is very sensitive to theconcentration of the oxygen in the primary air. That is, when thecontents of the oxygen is less than a predetermined level, the inner orprimary flame cone F₁ lifts off the mouth 7 of the inner tube 4 andblows out. Therefore the Smithell's burner may be combined with suitablemeans capable of detecting whether the inner or primary flame cone F₁exists or blown off so as to provide a device for detecting aninsufficient supply of oxygen. Thus accidents due to carbon monoxide maybe prevented.

However, the Smithell's burner has an inherent defect that thecombustion range in which both the primary and secondary flame cones F₁and F₂ may be securely established is very limited. As a result, a widevariety of nozzles must be provided for various types of gases, raisingan economic problem.

This defect will be described in some detail with reference to FIG. 4showing the combustion or burning characteristic curves. Burningvelocity is plotted along the ordinate while the equivalent ratio, alongthe abscissa. The equivalent ratio φ is defined as [(the volume ofgas)/(the volume of air)/(the volume of gas)/(the theoretical volume ofair)]. The "back curve" and the "lift curve" of the gas with a slowestburning velocity are indicated by A and A', respectively. Those of thegas with a highest burning velocity, by B and B', respectively. With theburner of the type shown in FIG. 3, the inner and outer flame cones F₁and F₂ may be established separately from each other in the region A" inthe case of the gas with a slowest burning velocity, while in the caseof the gas with a highest burning velocity, they may be established inthe region B". That is, depending upon the gas to be used, the region inwhich both the inner and outer flame cones F₁ and F₂ may be establishedsecurely and separately from each other varies.

FIG. 5 shows the flame cones depending upon the equivalent ratio φdefined above. The equivalent ratio φ or the amount of primary air isprogressively increased or decreased from FIG. 5(a) to FIG. 5(d). Atφ=1, all the gas burns at the mouth 7 of the inner tube 4 as shown inFIG. 5(a) so that no outer or secondary flame cone F₂ is formed. Whenthe ratio φ increases or when the amount of primary air decreases, theunburned gas emerges from the inner cone at the mouth 7 of the innertube 4 is supplied with the secondary air from the surroundingatmosphere at the mouth 8 of the outer tube 5 so that the outer orsecondary flame cone F₂ is formed as shown at (b) in FIG. 5. When theratio φ increases further, the amount of the primary air decreases sothat no inner or primary flame cone F₁ is formed at the mouth 7 of theinner tube 4 as shown at (c) in FIG. 5. Since the flow of the gas-airmixture is reduced at the mouth 8 of the outer tube 5, the flame cone F₂strikes back into the outer tube 5, but there is not a sufficient supplyof the oxygen in the outer tube 2 so that no flame cone is producedtherein. As a result, neither the inner or the outer flame cone isproduced as shown at (c) or (d) in FIG. 5.

When the ratio φ is further increased, no flashback occurs so that onlythe outer or secondary flame cone F₂ is produced as shown at (e) in FIG.5.

As described above, in the case of the conventional Smithell's burnerthe combustion region in which both the inner and outer flame cones F₁and F₂ may be produced in a stabilized manner is very limited so that itcannot be used with various types of gases.

THE INVENTION First Embodiment, FIGS. 6-21

Prior to the detailed description of the first embodiment with referenceto FIGS. 6-21, its features will be briefly described. It has a mainbody consisting of an inner tube and an outer tube. The mouth at whichis produced the outer or secondary flame cone is vertically upwardlyspaced apart by a suitable distance from the mouth of the inner tube atwhich is produced the inner or primary flame cone. In addition to thefuel gas and the primary air, auxiliary air is admitted into the innertube. The flow rate of the auxiliary air is so selected that the inneror primary flame cone F₁ may be produced in a stabilized manner withinthe range of the gradient of the boundary velocity from 7×10² -4×10³sec⁻¹ and within the range of the equivalent ratio φ between 1.1 and2.0. Of course the flow rate of the primary air fed into the inner tubeis so controlled as to maintain the above two conditions. Therefore witha single burner various types of gas fuels may be equally burned in sucha way that the inner and outer flame cones may be produced under theabove conditions. The burner of the present invention is thereforesatisfactorily used as a means for detecting the insufficient supply ofthe oxygen or air to the gas burner as will be described in detailhereinafter.

The dimensional data of the first embodiment of the gas burner inaccordance with the present invention are as follows:

    ______________________________________                                        The inner diameter of the mouth of the inner tube:                                                      4 to 5.5 mm                                         The length of the inner tube:                                                                           55 to 80 mm                                         The inner diameter of the mouth of the outer tube:                                                      8 to 20 mm                                          The difference in height between the mouth                                    of the outer tube and the mouth of the inner tube:                                                      20 to 40 mm                                         The diameter of the secondary air ports:                                                                3 to 4 mm                                           The distance between the mouth of the inner tube                              and the center of the auxiliary air port:                                                               5 to 30 mm                                          The distance between a thermocouple and the                                   inner or primary flame cone:                                                                            12 to 35 mm.                                        ______________________________________                                    

The gas burner with the above dimensions may be incorporated in thegas-fired water heater or the like and may burn any type of gas fuel. Asa result, it may be used as a general type detector capable of detectingan insufficient supply of the oxygen and hence the air, whereby theaccidents due to the decrease in the contents of the air in rooms may becompletely eliminated.

It is to be understood that instead of the thermocouple T describedabove, any suitable oxygen sensor means may be used. There are forinstance an oxygen concentration cell which utilizes a high temperaturesolid electrolyte which conducts oxygen ions, an oxygen partial pressuresensor such as titanium oxide, an ionic current detector and so on.

Now referring to FIG. 6, 11 is a burner main body consisting of theinner tube 12 and the outer tube 13. The outer tube 13 has the secondaryor outer flame cone mouth 14 at the upper end thereof, and the lowerperipheral wall thereof is formed with the auxiliary air port 15 throughwhich a small amount of air passes. Meanwhile the inner tube 12 isdisposed within the outer tube 13 and has the primary or inner flamecone mouth 16 at the upper end thereof which is located downwardly ofthe secondary or outer flame cone mouth and upwardly of the auxiliaryair port 15. The lower wall is formed with a primary air port 17. 18shows a gas nozzle attached to the lower opening of the inner tube 2.

Next the combustion operation of the above burner will be described. Thegas injected through the gas nozzle 18 and the primary air inducedthrough the primary air port 17 are sufficiently mixed within the innertube 12 and obtain a small amount of air flowing through the auxiliaryair port 15 and burn at its primary or inner flame cone mouth 16,thereby producing the primary or inner flame cone F₁ consisting ofan-inner flame cone F₁ ' and a outer flame cone F₁ ". The gas which hasnot burned here emerges out of the secondary or inner flame cone mouth14 at the leading end of the outer tube and obtains the air from thesurrounding atmosphere, or the secondary air, to burn, thereby producingthe secondary or outer flame F₂.

In the case of the separate flame combustion of the type describedabove, the primary flame cone F₁ which is dependent, is in the processof the instable combustion and is very sensitive to the contents of theoxygen because the supply of the air through the auxiliary air ports 15is controlled. And when the contents of the oxygen in the air dropsbelow a predetermined level, the primary flame cone F₁ blows off so thatonly the second flame cone F₂ remains.

Next the effects on the combustion of the auxiliary air will bedescribed with reference to FIG. 7.

That is, FIG. 7a shows that the equivalent ratio φ=[(the volume ofgas)/(the amount of air)/(the amount of gas)/(theoretical amount ofair)] is about 1. In this case, of course the primary or inner flamecone F₁ is produced only at the primary or inner flame cone mouth 16.

When the amount of air is decreased further so as to increase the ratioφ, the secondary or outer flame cone F₂ is produced at the secondary orouter flame cone mouth 14 as shown at b. Even when the ratio φ isincreased, the separated flames remain as shown at c and d. It is FIGS.7c and 7d that are different from FIGS. 5c and 5d which show the priorart examples. The difference resides in the formation of the outer flamecone F₁ " due to the supply of the auxiliary air.

That is, because of the existence of the outer flame cone F₁ ", the blowoff of the primary flame cone F₁ becomes difficult. Even when theequivalent ratio φ increases considerably, the primary flame cone F₁remains.

When the increase of the equivalent ratio φ continues, as shown at d inFIG. 7, the tip of the primary flame cone F₁ is torn off, and finally asshown at e the primary or inner flame cone F₁ blows off while only thesecondary flame cone F₂ remains.

These combustion or burning conditions may be expressed in terms of theburning characteristics as shown in FIG. 8. The separate flamecombustion regions A" and B" are enlarged. And when the equivalent ratioφ=1.1 to 2.0 and the boundary velocity gradient g=7×10² -4×10³, theseparate flame combustion or burning regions are overlapped so that whenthe equivalent ratio φ and the velocity gradient g are set within theabove ranges, the single burner can accomplish the separate flamecombustion of various types of gas fuels. In other words, the primaryflame cone F₁ may be blown off at a predetermined equivalent ratio φ orat a predetermined level of the oxygen contents.

In the burner wherein the primary and secondary flame cones areseparated, the oxygen deficiency sensor which is the thermocouple T inthis embodiment is disposed so that the electromotive force may begenerated by the temperature received from the primary or inner flamecone and in the case of the normal combustion an electromagnet valve isheld open. However, when the content of oxygen drops for any reason, theprimary flame cone 9 lifts and the thermocouple electromotive forcedrops so that the electromagnet valve is released.

The burner which operates in the manner described above respondsimmediately to variations in the content of the oxygen in the air eventhough the variation is very small and it burns in a stable manner inthe case of normal combustion. Furthermore, in order to utilize a widevariety of gas fuels without modification of the construction of theburner, some of the important dimensions must be determined. They arethe nozzle diameter l₁ which determines the combustion setting position;the diameter l₂ of the primary air port; the position l₃ of the primaryair port; the diameter l₄ of the primary flame cone mouth; the length l₅of the inner tube; the outer tube diameter l₆ which stabilizes theprimary flame cone and cuts off part of the outer flame, thereby causingthe quick change in the case of the oxygen deficiency; the difference inlength between the inner and outer tubes l₇ ; the diameter of theauxiliary air port l₈ ; the position of the auxiliary air port l₉ andthe distance l₁₀ from the thermocouple T to the primary flame cone.These will be described in more detail below (See FIG. 9).

When the dimensions for practical mounting on equipment are considered,there exists a suitable burner size or capacity as an oxygen deficientsensing pilot burner for a hot water heater, a stove or the like andthere exist dimensions corresponding to the suitable burner size. Inorder that it may be possible to ignite the main burner as a pilotburner which is normally used and from the view point of the pilotheating and other performances, it has been preferable to select 50 to200 KCal/h as a combustion quantity. Typical dimensions at which theflame may burn at the primary flame cone mouth at this combustionquantity are the diameter l₄ of the primary flame cone between 4 to 5.5mm (the opening area being 12.56 to 23.75 mm²) which are used asreferences. That is, when φ is less than 4 mm, I the nozzle diametermust be extremely reduced in the case of LPG having a high heating valueand II as shown in FIG. 10 in the case of a gas fuel with a low burningvelocity, the flame length suddenly increases less than 4 mm in diameterand the control of this flame length is difficult because of thetolerances and so on of the primary flame cone mouth diameter. On theother hand, when φ is in excess of 5.5 mm, the burning velocity is fastso that flash back occurs.

With the primary flame mouth thus determined, the flow rate of the mixedgases is determined for a specific type of gas fuel. When a suitableposition of the combustion of the primary flame cone is determined so asto separate flames, there must be a difference between the diameters ofthe inner and outer tubes. As shown in FIG. 11, the diameter at whichthe combustion continues in such a way that the burning occurs at thesecondary flame cone mouth while no lift occurs at the primary flamecone mouth is more than 8 mm in the case of the outer tube. In the caseof flash insufficient supply of the oxygen, the boundary velocitygradient of the gas is reduced so that the lift occurs at the primaryflame cone mouth. If this primary flame may be burned at the secondaryflame cone mouth 14, the sensitivity as the first oxygen deficiencydetection may be maintained because there exists no flame at the primaryflame cone mouth. Therefore flash back from the secondary flame conemouth 14 to the primary flame cone mouth 16 must be avoided. As aresult, as shown in FIG. 12, the region is such that the outer tubediameter is less than 20 mm. When the secondary flame cone mouthdiameter is in excess of 20 mm, even in the case of the oxygendeficiency, the flame goes up and down between the primary and secondaryflame cone mouths 14 and 16 so that the drop in the electromotive forceis delayed. As a result, the difference between the oxygen deficiencyand the normal combustion becomes small and the sensitivity is degraded.[(the area of the outer tube)/(the area of the inner tube)=(8/5.5)²-(20/4)² =2.12-25 ].

As shown in FIG. 13, the difference in height between the primary andsecondary flame cone mouths 16 and 14 must be longer than any of themaximum flame length of the primary flame cone because the behavior ofthe primary flame cone F₁ may be detected only by clearly separating theprimary and secondary flame cones F₁ and F₂, thereby detecting theoxygen deficiency (See FIG. 13 II). Furthermore, required is the lengthwhich is free from the temperature influence from the flame transferredto the secondary flame cone mouth 14 in the case of the oxygendeficiency to the thermocouple. That is, as shown in FIG. 14, themaximum length of the flame is of the order of 12 mm. When the length isincreased, the temperature variations of the flame are small, but asseen from FIG. 15, the influence of the flame produced at the outerflame cone mouth in the case of the oxygen deficiency arises when thelength is between 12 and 20 mm. As a result, the secondary flame conemouth 14 must be spaced apart from the primary flame cone mouth 16 bymore than 20 mm. However, when the secondary flame cone mouth 14 isspaced apart from the secondary flame cone mouth by too far a distanceso that the increase in length of the outer tube results, the draft isincreased so that the stability is degraged in the case of the normalcombustion. Thus, in practice, the length is preferably less than 40 mmso that the gas burner may be incorporated in the equipment.

The auxiliary air is proposed as means for producing the primary flamecone of any types of gas fuels at the primary flame cone mouth and forenclosing the primary flame cone with a thin outer flame cone, therebyattaining stability. However, when the auxiliary air is increased involume, as the variation in the electromotive force of the thermocouplein the case of the oxygen deficiency indicates, when the diameter of theauxiliary air port is increased beyond 4.0 mm, the electromotive forceof the thermocouple will not drop even in the case of the oxygendeficiency. As a result, the diameter of the auxiliary air port must bemade less than 4.0 mm so that the electromotive force of thethermocouple may be dropped prior to the generation of the CO from themain burner, thereby releasing the electromagnet valve so as to close agas circuit.

On the other hand, in order that the stability may be maintained in thecase of the normal combustion, as shown in FIG. 17, there must beprovided the electromotive force sufficient for not releasing theelectromagnet valve in the gas circuit even when the gas fuel is usedwhich tends to lift at a minimum burning velocity. According to theexperiments, it was found out that the auxiliary air port with thediameter l₇ of greater than 3 mm must be opened so that the range inwhich the electromotive force will not drop must be used.

Furthermore, this auxiliary air supply means is disposed below theprimary flame cone mouth 16 so that the flow from the below may beprovided and consequently the lift-off may be smoothly effected in thecase of the oxygen deficiency. According to the experiments, as shown inFIG. 18, when the position 1-8 of the auxiliary air port is spaced apartfrom the inner tube flame mouth position by 0-5 mm downwards thereof,the contents of the oxygen which lifts in the case of the oxygendeficiency becomes low so that the sensitivity is low. However, if it ismore than 5 mm, the lift may be smoothly effected. However, when theouter tube is extended too much below the primary flame cone mouth 16,the temperature rises because of the influence of the suction throughthe primary air port formed at the lower portion of the inner tube anddue to the heating of the outer tube by the primary flame. When theouter tube is extended, the temperature of the gas mixture flowingthrough the inner tube rises so that in the case of the gas fuel with ahighest burning velocity, there is a danger of resulting in theoccurence of flash back. Therefore, in practice the position of theauxiliary air port which is formed through the outer tube must be lessthan 30 mm below the primary flame cone mouth. Furthermore, thisdimension is limited by the length of the inner tube to be describedbelow. The so-called mixing tube through which the mixture of gas fueland air for producing the primary flame cone F₁ flows has been reportedto preferably have a diameter 8 times the throat diameter. However,according to the experiments, when it is 8 times, excessive fluctuationsof the primary flame cone result. As a result, the boundary velocitygradient increases so that the lift-off in the case of the oxygendeficiency is delayed. It must be more than 10 times as much as thediameter of the primary flame cone mouth and the inner tube must begreater than 55 mm in length so that the primary flame may form abeautiful laminar layer flame and may smoothly lift-off in the case ofthe oxygen deficiency. (In the case of the diameter of the inner tubebeing 5.5 and See FIG. 19). When the length of the inner tube is inexcess of 55 mm, it may have any length from the theoretical standpointwhen the drop in suction due to the passage resistance within the tubemay be covered. However, when it is incorporated into an equipment orthe like, the pilot burner is about 100 mm at the most in practice.Therefore it is preferable to use the inner tube less than 80 mm inlength.

When the dimensions are determined in the manner described above, it maybe used as an oxygen deficiency pilot burner capable of encountering anytype of gas fuels. However, the city or town gas, LNG and LPG havedifferent heating values so that when the heating value is to bechanged, the nozzles are changed depending upon the heating value. Andthe suction of the primary air is different depending upon the diametersof the nozzles. As a result, the damper adjustment must be made at theprimary air port depending upon the type of gas fuel used as before.

FIG. 20 shows an illustrative example wherein the burner of the typedescribed above is used as a hot water heater. That is, 19 is a heatexchanger and 20, a main burner. The burner main body 11 in accordancewith the present invention is disposed in the proximity of the mainburner. A branched gas line 22 branched from a gas line 21 to the maingas burner 20 is connected to the gas nozzle 18. 23 is an electromagnetsafety valve inserted in the gas line at the upstream of the branchedpoint of the branched gas line 22; and T, a thermocouple which is apower source for it, it being located above the primary flame cone mouth16 of the burner main body 11. 24 is a governer for controlling thecombustion by the main gas burner 20.

Normally, the burner main body 11 burns the gas fuel, forming theprimary and secondary flame cones. The thermocouple T is heated by thesaid primary flame to generate the thermal electromotive force, thusmaintaining the electromagnet safety valve 23 opened.

When the electromagnetic safety valve 23 is opened, the gas fuel issupplied to the main gas burner 20 and ignited by the secondary flame.

When the contents of the oxygen drops to a predetermined level due tothe contamination of air, the primary flame blows off so that nothermo-electric motive force is obtained from the thermocouple T. As aresult, the electromagnetic safety valve 23 is closed and consequentlythe supply of gas is interrupted.

With the above arrangement and operation, in order to encounter varioustypes of gas fuels, the relationship in position between thethermocouple and the primary flame cone becomes important. In the caseof the gas fuel with a faster burning velocity, the flame length isshort, but in the case of the gas fuel with a slow burning velocity, theflame length is longer.

FIG. 21 obtained from the experiments the relationship between thedistance S between the primary flame cone mouth 16 in the burner 11 (SeeFIG. 9) and the thermocouple T and the electromotive force E of thethermocouple T. Used in the tests were the gas G₁ with the fastestburning velocity; the gas G₃ with the slowest burning velocity; and thegas G₂ with an intermediate burning velocity.

With the distance S=5 mm and with the gas G₁, the electromotive force Eis high. With the gases G₂ and G₃, the electromotive force E is low.With the gas G₁, the flame length is short. This means that thethermocouple T is efficiently heated. However with the gases G₂ and G₃,the flame becomes longer in length and the thermocouple is positionedwithin the flame so that no efficient heating is made.

With the increase in the distance S, the difference in the electromotiveforce E between the gases G₁, G₂ and G₃ becomes less. With the distanceof longer than 12 mm, the variations are almost negligible.

However, when the thermocouple T is spaced apart from the primary flamecone mouth 16 by a distance of longer than 12 mm, the electromotiveforce becomes constant regardless of the type of the gas fuel used sothat the electromagnetic valves of the same specifications may beemployed.

The upper limit of the distance S is determined depending upon thethermoelectromotive force and the shapes of the burners, but in practiceit is preferably less than 35 mm.

Another embodiment of the present invention is shown in FIGS. 22 and 23.An outer tube 27 is extended from a fuel or flame element 26 of a Bunsentype main burner 25. An oxygen deficiency sensor 28 such as athermocouple may be extended through the outer tube 27. The outer tube27 is formed with an auxiliary air intake port 29. 30 is a nozzleholder; 31, a nozzle; 32, a gas supply pipe. When the sensor 28 detectsthe abnormal combustion, a valve 33 is closed to interrupt the supply ofgas so that an accident may be prevented. As described above, it ispossible to use a part of the main burner as an inner tube.

What is claimed is:
 1. An improved pilot gas burner of the Smithelltype, comprising:an inner tube having an inlet end for receiving amixture of combustible gas and primary air, and an open end comprising aprimary flame cone mouth having a predetermined cross-sectional area; anouter tube surrounding and coaxial with said inner tube adjacent andextending beyond said primary flame cone mouth thereof, said outer tubehaving (i) an auxiliary air inlet port in the wall thereof at a positionbetween said inlet end and primary flame cone mouth of said inner tube,and (ii) an open end adjacent said primary flame cone mouth comprising asecondary flame cone mouth having a given cross-sectional area; theratio between said given area of said secondary flame cone mouth andsaid predetermined area of said primary flame cone mouth being in therange of 2.12 to 25; and means for controlling the flow rates of saidcombustible gas, primary air and secondary air so that (i) primary andsecondary flame cones may be sustained at said primary and secondaryflame cone mouths respectively, (ii) the equivalent ratio respectingsaid combustible gas and primary air is in the range of 1.1 to 2.0, and(iii) the gradient of boundary velocity gradient of said combustible gasis in the range of 700 to 4,000 sec.⁻¹.
 2. The gas burner according toclaim 1 further comprising means for detecting the presence or absenceof said primary flame cone.
 3. A gas burner as defind in claim 1 whereinsaid auxiliary air port is formed through the wall of said outer tube ata position spaced apart by 5-30 mm from said primary flame cone mouth.4. A gas burner as defined in claim 1, wherein a portion of said innertube comprises a burner main body, and l>10d, where l is the length ofsaid burner main body and d is the inner diameter thereof.
 5. A gasburner as defind in claim 1, wherein the inner diameter of said primaryflame cone mouth of said inner tube is 4-5.5 mm; the length thereof is55-80 mm; said outer tube extends 20-40 mm; the diameter of said outertube is 8-20 mm beyond the open end of said inner tube; the diameter ofsaid auxiliary air port is 3.0-4.0 mm; and the position of saidauxiliary air port is spaced apart from said primary flame cone mouth ofsaid inner tube by 5-30 mm.
 6. A gas burner comprising an inner tube, anouter tube disposed so as to surround said inner tube, and an auxiliaryair supply means formed at a part of said outer tube; means forsupplying a mixture of primary air and a combustible gas from one end ofsaid inner tube so as to produce a primary flame cone at a primary flamecone mouth at the other end of said inner tube; means comprising saidauxiliary air suply means for supplying auxiliary air to said outer tubeso as to produce a secondary flame cone at a secondary flame cone mouthat one end of said outer tube; the amounts of said primary air, saidauxiliary air and said gas being so controlled that the combustioncharacteristics of said primary flame cone may be attained with aboundary velocity gradient in the range of 7×10² -4×10³ sec⁻¹ and anequivalent ratio in the range of 1.1-2.0; and an oxygen deficiencysensor is provided which operates in response to the detection of theabnormal burning of said primary flame cone so as to interrupt thesupply of said gas.
 7. A gas burner as defined in claim 6, wherein saidoxygen deficiency sensor is a thermocouple.
 8. A gas burner as definedin claim 6, wherein said oxygen deficiency sensor comprises an oxygenion conductive, high-temperature, solid electrolyte.
 9. A gas burner asdefined in claim 6, wherein said oxygen deficiency sensor comprises anoxygen partial pressure sensor.
 10. A gas burner as defined in claim 6,wherein said oxygen deficiency sensor comprises an ion current sensor.